This invention relates to starting devices for high intensity discharge (HID) lamps and, more particularly, to a universal two-lead igniter useful in the ignition of metal halide and other HID lamps.
Various forms of presently available HID lamps, especially metal halide lamps, require voltage spikes on the order of several thousand volts in order to produce reliable ignition of the lamp. In order to generate these very high voltages economically, various electronic devices and circuits have been designed and are employed commercially at the present time. The energy transformation technique used in these conventional lamp ignition devices generally utilize the coil and/or capacitor of the lamp ballast apparatus in order to step up the available 60 Hz AC line voltage to the KV range.
FIG. 1 of the drawing illustrates a common form of lamp igniter circuit in general use today which will be discussed in greater detail below. Briefly, the capacitor (11) gradually charges up to the voltage breakdown level of the Sidac (13), whereupon the capacitor discharges rapidly through the Sidac and a small part of the transformer secondary winding. This voltage is stepped up via the transformer to develop a large voltage, i.e. several KV, across the entire secondary winding and hence across the lamp to produce lamp ignition. A disadvantage of this starter-igniter device is that the high-voltage collapses fairly rapidly which makes the lamp ignition less than entirely reliable. This problem can be minimized by modifying the ballast-igniter circuit in the manner described in U.S. Pat. No. 4,695,771 (9/22/87) in the name of Alexander Hallay.
U.S. Pat. No. 4,339,695 describes a high pressure sodium (HPS) lamp ballast circuit (J. V. Siglock) that utilizes a conventional igniter to start a high pressure discharge lamp. This circuit requires a pulse auto-transformer with a tap point in order to generate a high voltage ignition pulse in the order of 2500 to 4000 volts. FIGS. 1 and 2 of this patent show an igniter consisting of the pulse auto-transformer, a Sidac solid state switch, a capacitor and a resistor which is used to charge the capacitor. A clamping circuit is coupled to the igniter and consists of eight components (FIG. 1) or six components (FIG. 2). The clamping circuit is required in order to limit the VA rating of the ballast which operates the HPS lamp during starting, hot restart, and lamp out conditions.
The pulse auto-transformer used by Siglock is connected in series with a lamp across the secondary winding of the ballast. The large number of components makes the whole system uneconomical and by adding power losses thereto it also makes the system relatively inefficient. In the case where the lamp is not connected, or during the initial starting phase, the Siglock igniter produces a single voltage pulse near the peak of the open circuit waveform of the secondary winding output voltage, but not at the lower end thereof (FIG. 3 of Siglock).
Another starting circuit for discharge lamps is shown in U.S. Pat. No. 3,758,818 by I. Kaneda. This starting circuit uses two closed circuits that share a common capacitor. The first closed circuit includes a power source and an inductive stabilizer in combination with the capacitor. The second closed circuit includes an inductor and a bidirectional diode thyristor in combination with the capacitor. A second bidirectional diode thyristor having a lower break-over voltage than the first one is provided and constitutes, along with the second closed circuit, a starting circuit for the lamp. In the second closed circuit, the capacitor is charged via the second bidirectional diode thyristor to the instantaneous value of the power source and is discharged by the first bidirectional diode thyristor through the inductor which produces a high voltage pulse which is applied to the lamp to start it. The output voltage waveform shows a high frequency oscillation only at the top portion of the open circuit voltage waveform. Disadvantages of this circuit are the requirement for two switching devices and the large number of circuit components which makes it expensive and less efficient.
A further discharge lamp starter device which uses a backswing voltage booster is described in U.S. Pat. No. 3,866,088 by Kaneda et al. This starter circuit consists of the backswing voltage booster which includes a capacitor connected across the lamp for oscillation, a series circuit of a saturable non-linear inductor and a bidirectional diode thyristor, and a current limiting capacitor connected across the non-linear inductor. The output of the booster is in the form of an oscillating voltage which is produced by the switching action of the bidirectional diode thyristor together with the operation of the capacitor discharging through the non-linear reactor that produces the high voltage pulses that appear across the common capacitor producing oscillation of the output voltage from the power source. The Kaneda et al starter device requires the presence of a non-linear inductor. The cost of such a starter device is relatively high and the circuit efficiency is not optimum.
Furthermore, in the Kaneda et al circuit, if the discharge lamp is inadvertently removed, the continuous oscillation and the magnitude of the boosted voltage will cause damage or destruction of the circuit components if the non-linear inductor is not biased properly. The output to the discharge lamp can be controlled by a bias means for negatively or positively exciting the magnetic field of the core of the saturable non-linear reactor. Therefore, an added bias coil winding has to be provided for fail safe protection in order to limit the oscillating period and the magnitude of the boosted voltage.